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Abstract

The second-order photon correlation function g(2)(τ) of photons emitted by a continuously pumped ensemble of N two-level systems coupled to a single-mode optical cavity well below the lasing threshold is investigated theoretically. A giant photon bunching is found for N < 10 emitters as the microscopic counterpart of spontaneous emission noise driven quasi-periodic superradiant pulse sequences in macroscopic systems of large numbers of emitters N ≫ 1. The phenomenon of giant photon bunching is preserved even for N = 2 and can be explained by the cooperative evolution via dark and bright two-atom states resulting into emission of superradiant photon pairs. The sensitivity of g(2) to microscopic dephasing processes and resonance frequency detuning opens the door for photon bunching spectroscopy.

Second-order photon correlation function g(2)(τ) for N incoherently pumped two-level systems (γpump = 0.01) for different values of dephasing rate γdeph. For N=1 anti-bunching is observed, which does not depend on dephasing. For N > 1 a pronounced bunching is observed, which is extremely sensitive to γdeph. Inset shows the dependence of bunching amplitude g(2)(0) on N. All rates are in units of g2/κ.

Level structure for two two-level systems. The bunching pathway after simultaneous excitation of both atoms via a subradiant (dark) state leads to emission of photon pairs via a superradiant (bright) state. See text for details.